The Case of the Smoky UPS Cabinet

Some time ago I was asked to commission the SCADA on a substation in a desert town in Saudi Arabia. I knew our products pretty well but there was one part that worried me: a very large UPS with Nickel Cadmium batteries. All the equipment had been shipped from Australia and installed prior to my arrival by extremely competent local technicians. After very apprehensively filling the batteries with the very nasty electrolyte, I was ready to turn on the charger.

The charger controller comprised a circuit board that mounted on the back of the UPS cabinet door, the component side facing you as you looked at the back of the door. There was a cutout in the door that allowed LED displays that were mounted on the “back” of the PCB to project through and be seen from the front of the cabinet. A square of red tinted acrylic was mounted on the front of the door to hide the PCB from view. The bolts that held the acrylic panel in place passed through the door and also held the PCB in place.

I had not commissioned battery chargers before, so I was very nervous when I turned on the power. To my horror there was a flash and a cloud of smoke from the PCB. My worst nightmare. I immediately turned off the power and tried to work out what had happened. I removed the PCB from the door and inspected it for damage. All that was obvious was that there were two resistor legs on the PCB surrounded by soot and no resistor. I had no way of knowing what the value was or what it did. The company did not provide schematics. I investigated the reason for the problem and discovered that, when the PCB had been mounted on the door, the spacers that were supposed to stop it from touching the powder-coated door were placed on the wrong side of the door and were, in fact, on the acrylic panel side. All had been well during FAT because the powder coating had insulated the PCB from the door. In transit the sharp points of the components on the PCB had scratched through the paint causing some to short out. This was not very helpful.

Since commissioning could not take place without the charger operating, I had to fix it. I was 350km from any major city that might have a store at which I could buy components - assuming I knew what to get. I rang my office in Australia to get the phone number of the manufacturer so I could discuss the problem. The technician at the UPS company was extremely sympathetic and helpful. He said that they never give out schematics but in my case he would make an exception. The technicians working with me arranged to borrow a fax machine and the schematics were faxed through. I examined the circuit and discovered that the missing resistor was 100ohm ¼ watt. Now the problem was where to get one. Again the local technicians came to the rescue and managed to salvage a 100 ohm ½ w resistor from an old TV. I was not very hopeful that this was the only problem but I had to start somewhere.

I soldered the resistor on the PCB and mounted it on the door with the spacers in the correct position. Very nervously I turned on the power. Imagine my relief when the UPS all started up perfectly.

Jim Baker has a degree in Electronic Engineering from Curtin University in Perth and has been involved in SCADA as a development/commissioning/training engineer for over 20 years.

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